The pictures left and below show my installation of a motorised fan in a Hornby class 50. The motor and gears come from a CD player sold for just such uses on a stand at a Model Railway exhibition. The original Hornby fan is used and fan and motor mounted to a simple plasticard frame which is a direct replacement for the Hornby original. Power comes from an auxiliary output of the DIY decoder built from a MERG kit. Click on the left hand photo to enlarge, it could be crisper but you should be able to make it out.